Ok, this is from my personal experience, owning both a 5.1 set on my computer and on my standalone DVD player in my little Home Theatre.
5.1 sounds *awesome* for DVD's and such ONLY IF it is set up correctly. That means the front speakers are a fair distance away from each other (not such on the side of your monitor) and the rear speakers are set up properly behind you. So, unless you have the space to do it properly (which on my computer i dont, making my 5.1 set useless) then go for something smaller but better for the same price.
Also, a 5.1 set up is just that, a set up. It wont have a significant effect unless your computer has the ability to play Dolby Digital or even better DTS soundtracks. If you really want it to sound more 'surrounded' then you must have a 5.1 capable soundcard (SB Live! 5.1, Audigy, Hercules GTXP - though only when using powerDVD etc.), and a DD/DTS decoder with your speakers.
Personally I find it easier and better to set up a 5.1 system on a standalone DVD player in your living room, though that is subective, and some people may not be able to do that.